1. Field of the Invention
The present invention relates to a pressure control apparatus in a vehicle braking system having a master cylinder which generates a hydraulic pressure in response to depression of a brake pedal.
2. Description of the Prior Art
In general, a hydraulic braking system for an automotive vehicle is provided with a master cylinder for generating a master cylinder pressure in response to depression of a brake pedal, a reservoir for supplying a brake fluid to the master cylinder, and a wheel cylinder for braking a wheel of the vehicle with the master cylinder pressure, as disclosed in Japanese Patent Laid-open publication No. 2-175361, for example. Between the master cylinder and the wheel cylinder, is filled the brake fluid which is supplied from the reservoir. When the brake pedal is depressed, a pressure chamber in the master cylinder is pressurized to generate the master cylinder pressure, and transmit it to the wheel cylinder, which applies a braking force to the wheel to stop the vehicle.
In contrast to the above ordinary braking system, it has been proposed to provide an apparatus for braking a vehicle automatically without depressing the brake pedal, as disclosed in Japanese Patent Laid-open publication No. 58-80800, for example. This apparatus is normally used for preventing the vehicle from slipping, or used for braking the vehicle automatically to keep a certain distance in front of the vehicle from other vehicles. In that apparatus, there is provided a pressure supply device such as an accumulator for accumulating a pressurized brake fluid, or a fluid pump for pressurizing the brake fluid, and discharging it to the wheel cylinder.
According to the vehicle having the above automatic braking apparatus, however, in the case where the braking force is applied to the wheel without depressing the brake pedal, i.e., during the automatic braking operation, if the brake pedal is depressed, the braking operation may not be made in response to depression of the brake pedal, or the braking force applied to the wheel may change abruptly.